Nine standardized cat extracts prepared from differing source materials were evaluated for compositional differences in content of non-Fel d I allergen content by means of parallel line skin test. All extracts were compared to a reference cat allergen with a high content of non Fel d I allergen. All cat extracts were standardized based on in vitro content of Fel d I allergen. All cat extracts were tested in two populations of cat allergic subjects: a Fel d I allergic population and a non Fel d I allergic population. Cat allergic subjects whose reactivity to non Fel d I allergens exceeded their Fel d I reactivity accounted for only 7% of all cat allergic subjects tested. In Fel d I reactors the correlation between relative potency based on Fel d I content and in vivo relative potency was r=0.953, p0.0001. In contrast, in non Fel d I reactors this correlation was r=0.318, p=0.40. Compositional differences in cat extracts were determined based on whether each extract was equipotent to the cat reference in the Fel d I and non-Fel d I population. All extracts in the Fel d I reactors were equipotent at equal Fel d I. Five of the extracts were significantly lower in potency than the reference in the non-Fel d I reactors. In vitro data suggested these extracts had lower levels of non-Fel d I allergens compared to the reference. A significant association between the ratio of Fel d I to total protein of the test and reference extract and the ratio of the in vivo relative potency of the test and reference in Fel d I reactors/non-Fel d I reactors was found, r=0.87, p=0.0025. Further study of the compositional differences among standardized cat extracts is ongoing.